Diethyl thiophosphoryl chloride and dimethyl thiophosporyl chloride are intermediates in the synthesis of the insecticides parathion and methyl parathion, respectively. Thus, the manufacture of these intermediates has been of great commercial interest, and a number of process innovations have been disclosed. The routes to dialkyl phosphorochloridothioates include both one-step and two-step processes. Phosphorous pentasulfide, alkyl alcohol and chlorine generally are the starting materials for both types of process. The overall reaction can be represented: ##STR1## in which R is an alkyl group.
In the one-step process, phosphorus pentasulfide, alcohol and chlorine are reacted together to prepare the ester corresponding to the alcohol, and then the solvent is removed and the product separated. Typical one-step processes are disclosed in U. S. Pat. Nos. 3,356,774 and 3,502,750.
U.S. Pat. No. 3,356,774 discloses reacting a phosphorous pentasulfide suspension in an inert solvent at a temperature within the range of about 0.degree. C. to about 150.degree. C. with chlorine and an alcohol having 1-6 carbon atoms. In this process a stream of chlorine is introduced into the suspension, and the alcohol is added dropwise concurrently. When the chlorination reaction has proceeded to completion, the solvent is removed, and the dialkyl phosphorochloridothioate is recovered by distillation.
According to U.S. Pat. No. 3,502,750 lower alkyl esters of phosphorochloridothioic acid are prepared by reacting chlorine with a lower alkyl ester of dithiophosphoric acid and eliminating the harmful sulfur monochloride by-product from the reaction mixture by reacting it with hydrogen sulfide. The hydrogen sulfide preferably is that produced during production of the dithiophosphoric acid ester. The desired product is recovered by distillation.
According to the two-step process, in the first step, phosphorus pentasulfide is reacted with an alcohol, e.g., ethanol, to produce a dialkyl dithiophosphoric acid, e.g., diethyl dithiophosphoric acid, and hydrogen sulfide. In the second process step, the dialkyl dithiophosphoric acid is chlorinated in an appropriate solvent, such as toluene, with chlorine gas. The product is a dialkyl thiophosphoric acid chloride. The two-step process is described in U. S. Pat. Nos. 3,836,610 and 3,856,898, for example.
According to U.S. Pat. No. 3,836,610 the dialkyl dithiophosphoric acid intermediate is chlorinated, and the reaction mixture is then held at a temperature in the range of 85.degree. C. to 110.degree. C. until it is substantially free of sulfur monochloride, and the sulfur which is produced thereby becomes more thermally stable. The dialkyl thiophosphoryl chloride can then be readily and safely recovered from the reaction mixture by distillation.
According to U.S. Pat. No. 3,856,898 a mixture of a di(C.sub.1 -C.sub.8 alkyl) dithiophosphoric acid and amorphous sulfur, which can be present at up to about 1/3 the weight of the acids, is reacted with chlorine. The reaction mixture then is heated to a temperature at which substantially all of the sulfur goes into solution, but the dialkyl phosphorochloridothioate does not decompose. The reaction mixture is then cooled to a temperature at which the dissolved sulfur crystallizes from solution. The crystallized sulfur is then separated from the reaction mixture by filtration, centrifuge, or the like. The supernatant liquid containing the dialkyl phosphorochloridothioate is then distilled.
Prior art which discloses techniques for recovering the desired dialkyl phosphorochloridothioate from the reaction mixture also includes U.S. Pat. No. 3,897,523, which describes a process in which the crude dialkyl phosphorochloridothioate is vaporized in a film evaporator, the vapor is condensed, washed with water at 10.degree. C. to 50.degree. C., and the organic and aqueous phases are separated. The organic phase, which contains the desired dialkyl phosphorochloridothioate, is then dried. U.S. Pat. No. 4,025,586 discloses distilling the dialkyl phosphorochloridothioate product and washing the distillation residue with water to hydrolyze impurities. The washed residue, containing bis(thiophosphine)-sulfide, is then dried and recycled to the chlorination step. U.S. Pat. No. 3,089,890 discloses treating the crude distilled dialkyl phosphorochloridothioate with water, separating the organic phase, and drying it, which leads to the recovery of substantially contaminant-free phosphorochloridothioate. U.S. Pat. No. 4,159,289 describes a process for removing sulfur impurities from dialkyl phosphorochloridothioates by distillation in the presence of a naphthalenic liquid hydrocarbon, which solubilizes or suspends the sulfur.
In the conventional processes for producing dialkyl phosphorochloridothioates various impurities are produced, and the desired thioates generally are separated from the impurities by distillation. Distillation of the reaction mixture leaves a residual, still bottoms material which contains unstable phosphorus compounds, acids, and organic high boilers.
Although treating the still bottoms residue with aqueous strong base is a convenient technique for neutralizing, stabilizing and phase-separating the acidic and phosphorus- containing by-products from the organic high boilers, the aqueous phase still carries substantial quantities of phosphorus- containing materials. These include, for example, alkoxylated derivatives of the salts of hypophosphorous acid (H.sub.3 PO.sub.2), phosphorous acid (H.sub.3 PO.sub.2), phosphorous acid (H.sub.3 PO.sub.3), metaphosphorous acid (HPO.sub.2), hypophosphoric acid (H.sub.2 PO.sub.3), phosphoric acid (H.sub.3 PO.sub.4), metaphosphoric acid (HPO.sub.3), and pyrophosphoric acid (H.sub.4 P.sub.2 O.sub.7), as well as sulfur-containing analogs thereof in which one or more of the oxygen atoms is replaced with sulfur. Deep well injection can be used to dispose of the aqueous phase, but this is costly and potentially damaging to the environment. Furthermore, no value is realized from these by-products when they are simply discarded. The organic high boiler phase, on the other hand, can be utilized as fuel.